I upgraded my iPhone 11 to iOS 14 today and got the message “iPhone is not available. Please reconnect the device.” when trying to send an app to it from XCode 11.4.1.
The message doesn’t really match the problem, as the problem was actually that I needed to install XCode 12 in order to support iOS 14. XCode 11 doesn’t support iOS 14.
Quick fix; install XCode 12